Investigating Intermolecular Interactions in a DME-Based Hybrid Ionic Liquid Electrolyte by HOESY NMR

Abstract

The intermolecular interactions in a hybrid electrolyte based on various compositions of the ionic liquid N-methyl-N-propyl pyrrolidinium bis-fluorosulfonylimide (C3mpyrFSI), LiFSI salt and an ether-based additive, 1,2-dimethoxy ethane (DME), have been investigated using the HOESY (Heteronuclear Overhauser Effect SpectroscopY) NMR experiment. This NMR technique allows a quantification of the intermolecular interactions in ionic liquids (ILs) by measuring the cross-relaxation rate (σ) between different pairs of nuclei. Thereby, we compare the cross-relaxation rates between the cations, anions and DME in these hybrid electrolyte systems using 1H-7Li and 1H-19F HOESY experiments, and interpret the measured parameters in terms of ionic and molecular associations. The results give insights into the local coordination environment of the Li+ cations and their solvation by the FSI anions and DME.

The Intermolecular NOE Depends on Isotope Selection: Short Range vs Long Range Behavior

“…17,18 The elegant (yet not straightforward) fit of suitably normalized HOESY build-up curves with a modified expression including both the longitudinal relaxation times and diffusion coefficients allows for the precise calculation of absolute intermolecular crossrelaxation rates and their comparative use between different ionic liquids, concentrations, or temperatures. 19,20 The following calculations are based on a 500 MHz 1 H NMR spectrometer (cf. eq 3):…”

“…In this instance, we propose that the heteronuclear NOE of 1 H (γ H = 42.577 MHz T –1 ) and 7 Li (γ Li = 16.546 MHz T –1 ) is of a shorter range than the one between 1 H and 19 F (γ F = 40.078 MHz T –1 ). This theory was tested by O’Dell and co-workers using a combination of MD simulations and quantitative HOESY analysis. , The elegant (yet not straightforward) fit of suitably normalized HOESY build-up curves with a modified expression including both the longitudinal relaxation times and diffusion coefficients allows for the precise calculation of absolute intermolecular cross-relaxation rates and their comparative use between different ionic liquids, concentrations, or temperatures. , …”